Interlayer Coordination of Pd–Pd Units in Exfoliated Black Phosphorus

The chemical functionalization of 2D exfoliated black phosphorus (2D BP) continues to attract great interest, although a satisfactory structural characterization of the functionalized material has seldom been achieved. Herein, we provide the first complete structural characterization of 2D BP functionalized with rare discrete Pd2 units, obtained through a mild decomposition of the organometallic dimeric precursor [Pd(η3-C3H5)Cl]2. A multitechnique approach, including HAADF-STEM, solid-state NMR, XPS, and XAS, was used to study in detail the morphology of the palladated nanosheets (Pd2/BP) and to unravel the coordination of Pd2 units to phosphorus atoms of 2D BP. In particular, XAS, backed up by DFT modeling, revealed the existence of unprecedented interlayer Pd–Pd units, sandwiched between stacked BP layers. The preliminary application of Pd2/BP as a catalyst for the hydrogen evolution reaction (HER) in acidic medium highlighted an activity increase due to the presence of Pd2 units.


Supplementary Figures and Tables
. EELS spectra Figure S2. Raman statistics Figure S3. Pd 3d XPS spectrum of 1 Table S1. Pd 3d XPS data Figure S4. Pd 3d and P 2p XPS spectra of Pd2/BP 6% Figure S5. P 2p XPS spectrum of pristine 2D BP Figure S6. Time evolution of the P 2p XPS spectrum of Pd2/BP 6% Figure S7. Survey XPS spectrum and Cl 2p core level XPS spectrum of Pd2/BP 6% Figure S8. Solid state MAS NMR spectra Figure S9. ATR-FTIR spectra 1. Synthesis and Catalysis Figure S1. Comparative EELS nanoanalysis of Pd2/BP 3% (a) and Pd2/BP 6% (b). (c) EELS spectra acquired in Pd-rich regions (highlighted areas 2 and 4) and Pd-poor regions (highlighted areas 1 and 3). EELS spectra in (c) have been shifted on the vertical scale for the sake of comparison. The region around 200 eV highlighted with vertical bars comprises the Cl L-edge (no modifications observed). Figure S2. Statistical Raman analysis of Pd2/BP 6% and pristine BP based on 60 measurement points; the spectra are normalized to the silicon band. Ensemble of Raman mapping spectra of Pd2/BP 6% (a) and pristine BP (b) and their superposition (c). Intensity and Raman shift standard deviation of the Raman bands in Pd2/BP 6% (d) and pristine BP (e). Though a larger dispersion was observed for each phonon mode in Pd2/BP compared to pristine BP, on average no frequency shift was observed. Figure S3. Core level Pd 3d XPS spectrum of 1.  Figure S4. Core level Pd 3d (a) and P 2p (b) XPS spectra of Pd2/BP 6%. In the P 2p deconvolution, the green component (P 2p3/2 = 132.2 eV) is attributed to the P-Pd interaction, whereas the higher energy component in red (P 2p3/2 = 133.9 eV) to surface POx species resulting from oxidation. The red component (Pd-O) amount to about 7 % of the whole integrated area. Figure S5. Core level P 2p XPS spectrum of pristine 2D BP. Figure S6. P 2p XPS spectrum of Pd2/BP 6% freshly prepared (left) and after 12 h of air exposure. Percentages below the deconvoluted peaks represent the relative weight of each component. After 12 h, the more oxidized species in grey had increased at the expense of pristine BP (yellow component) from 6 % to 10 %. In contrast, the green component remained constant. Figure S7. Survey XPS spectrum (left) and core level Cl 2p spectrum (right) of Pd2/BP 6%. No chlorine was observed within the instrumental detection limit. The Au signal in the survey spectrum is due to an internal standard. Figure S8. Solid State NMR spectra of 2D BP, Pd2/BP 3% prepared from 1*, and Pd2/BP 6% prepared from 1.    . Optimized geometry of the Pd2 unit adsorbed on a bilayer BP surface. The adsorption of a dinuclear palladium unit on top of the BP surface has been taken into account and investigated with ab initio modelling. The optimized structure features a Pd-Pd distance of 3.05 Å, together with the coordination of each Pd center with three phosphorus atoms. However, this structure has been discarded for both an unfavourable structural arrangement and for being less stable than the model in Figure 5 by +37.0 kcal mol -1 .    Whereas the morphology of BP is only slightly affected, after 2 days major alterations occur to Pd2/BP flakes, looking mostly degraded and covered with large water blobs. Notably, upon degradation of the BP lattice in Pd2/BP 6% metal aggregation takes place, giving rise to several clusters of particles noticeable in the STEM micrographs (presumably made of Pd or PdO).    11.3 mm), then a controlled amount of deoxygenated water was added and the ampoule was sealed under inert atmosphere. The ampoule was dipped inside an ultrasonic bath (37 kHz, 80% power) and sonicated over 6 days at 30°C. After this time, a dark brownish dispersion was obtained. As probed via AFM and TEM analysis, the resulting flakes have lateral dimensions within the range 300-900 nm and average thickness below 30 nm. Prior to functionalization, the exfoliated material was washed to eliminate DMSO.

S5
In detail, the suspension resulting from sonochemical exfoliation (5 mg/5 mLDMSO) was centrifuged at 9500 rpm for 30 minutes to isolate 2D BP as a solid residue (the supernatant was discarded). The solid was resuspended in deaerated ethanol using ultrasounds (5 min), before being recovered after centrifugation. This washing cycle was repeated 4 times in total, using acetone in the 4 th step. The solid was dried under vacuum for 24 h, before an additional final washing with distilled and deaerated DCM was performed.

Synthesis of [Pd(C3H5)
Cl]2 (1). Complex 1 was prepared according to a literature method S5  2.4 Synthesis of [Pd(1-13 C -C3H5)Cl]2 (1*). The synthesis was carried out in two steps: first, the labelled allyl alcohol-1-13 C (AA*) was converted in the 13 C-enriched allyl chloride-1-13 C (AC*) and, second, the 13 Clabelled dimer 1* was synthesised from AC* (scale-down of the procedure 2.3). Since the overall purchased AA* amounted to 100 mg (~117 µL), a straightforward synthetic protocol to convert AA* in AC* avoiding any intermediate purification step (i.e. distillation) was necessary, which imposed to revisit S17 and improve a patented procedure. S6 The whole protocol was set up and checked using non-labelled AA before repeating the synthesis with AA*.
Step 1) 11 mg of PdCl2 (0.062 mmol, used as catalyst) were added to a short NMR tube, used as reactor, followed by 0.56 mL of concentrated HCl 37%. Once the solid was dissolved, 100 mg of AA* (1.72 mmol, [AA*] 1/2 /[HCl]= 0.19) were added, the tube was sealed, a customized refrigerator was mounted on top (see Figure S20) and the solution was heated to 80°C for 1 h. During this time, a colourless organic phase (AC*) was formed which separated above the reddish acidic phase.
Step 2) A glass vial equipped with a magnetic stirring bar was charged with 90 mg of PdCl2 (0.51 mmol, 1 eq) and 68 mg of NaCl (2.3 eq), followed by 410 µL of distilled water. The mixture was stirred until the palladium salt was dissolved forming soluble Na2PdCl4. Then, the acidic red phase at the bottom of the NMR tube was removed by syringe and discarded. 1.5 mL of MeOH were added to rinse the tube collecting AC* and then transferred to the vial containing Na2PdCl4. Further 1.0 mL of MeOH was added, then CO was slowly bubbled in the solution under stirring over 1 h (see Figure S20). A colourless precipitate formed (NaCl) while the solution turned yellow. The suspension was then poured into water (13 mL) and extracted with DCM (3 x 5 mL). The organic phases were collected altogether and washed with water (2 x 6 mL), then left 1 h over anhydrous MgSO4. The organic phase was concentrated to ca.
10 mL under a nitrogen stream and filtered over Al2O3 (via a Pasteur pipette packed with ~ 1 cm of Al2O3).
The filtrate was reduced to a small volume and n-pentane was added to precipitate 1* as a yellow crystalline powder (71 mg, 76% yield vs initial PdCl2).  H NMR (CD2Cl2, 400 MHz, 298 K). The 1 H NMR spectrum is reported in Figure S21a. 13 C labelling of a terminal atom in the allylic moiety turns the allylic A2E2M spin system of 1 into a second order AA'EE'MX system (see the labelling scheme depicted in the Figure). A simulation of the experimental spectrum was carried out with SpinWorks 4, with the following J (Hz) values: 13 C{ 1 H} NMR (CD2Cl2, 400 MHz, 298K). The 13 C NMR spectrum is shown in Figure S21b.   Ink preparation. The ink was prepared in a glass vial suspending the catalyst (2D BP or Pd2/BP) in DCM with ultrasounds for 1 min in cold water to slow down DCM evaporation during ink deposition. The suspensions concentration spans between 3.2 to 3.5 mg mL -1 and before each deposition the inks were resuspended with ultrasounds for 1 min.

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3. Characterization of the material 3.1 Inductively coupled plasma-Atomic emission spectroscopy (ICP-AES). ICP-AES measurements were performed with an Agilent 7700 Series spectrometer at the Chemistry Department, University of Florence (Italy). Samples followed a microwave-assisted digestion in nitric acid for trace analysis. Then, different dilutions of each sample with water for trace analysis were prepared, in order to obtain concentrations in the sensitivity range of the instrument for the elements under investigation (namely Pd and P). Standards at different concentrations were also prepared and measured contextually to sample measurements, in order to obtain a calibration curve for each element under investigation.  A GIF Quantum ER as Electron Energy Loss Spectrometer (EELS) was used for EELS measurements. Both low-and core-loss EELS spectra were acquired with the DualEELS capability through Gatan Digital

TEM microscopy. Transmission Electron
Micrograph software, which allows the accurate energy calibration of EELS spectra, thanks to the simultaneous alignment of the zero-loss peak position for every single acquisition which removes any artefact coming from energy shifts. The use of Fourier logarithmic deconvolution on a full spectrum obtained by splicing together low-and core-loss EELS allows removing thickness-related plural scattering. All the STEM-EELS and STEM-EDS measurements were performed simultaneously by using the Gatan spectrum imaging (SI) tool.

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3.5 Powder X-ray Diffarction (XRD). Data were collected with an X'Pert PRO diffractometer, operating in Bragg-Brentano parafocusing geometry with Cu-Kα radiation (λ = 1.5418) at 40 kV and 30mA. Samples were prepared slowly drop-casting the material suspended in DCM, while directing a nitrogen stream onto the sample holder to speed up solvent evaporation. The process was continued until a uniform film of the material had formed. Data acquisition was carried out under air exposure.
3.6 Raman scattering. Raman measurements were carried out at CNR-IFAC (Florence, Italy) using a micro-Horiba Xplora system coupled to a 532 nm wavelength laser. The backscattered light was collected by a 100× microscope objective with 0.9 NA, which generates a ~1 μm large laser beam waist. Integration times of 10 s, laser power values in the 1-2 mW range and a grating of 1200 cm -1 were employed. The samples were prepared by dropcasting a suspension of 2D BP or Pd2/BP in DCM on a Si/SiO2 wafer. After one minute of exposure, the wafers were rinsed with DCM and dried under a stream of nitrogen for 15 min. The Raman spectra displayed in Figure 3 were obtained averaging the data recorded from 15 individual flakes randomly chosen from each dropcasted sample. A second and larger statistical analysis was also carried out using 60 spectra for both Pd2/BP and pristine BP. The latter were collected choosing 60 random points from larger (ca. 20 µm in diameter) multi-flake aggregates on the silicon wafer. The results of this analysis are reported in Figure S2.
3.7 ATR-FTIR. Attenuated total reflectance (ATR) FT-IR spectra were recorded under air with a Perkin-Elmer Two Spectrometer, equipped with an ATR unit with diamond crystal. Spectra acquisition was carried out with a resolution of 4 cm −1 using 64 scans.
3.8 X-ray Photoelectron Spectroscopy (XPS). X-ray Photoelectron Spectroscopy (XPS) measurements were performed at the Chemistry Department, University of Florence (Italy) in an ultra-high vacuum (10 -9 mbar) system equipped with a VSW HAC 5000 hemispherical electron energy analyzer and a non-monochromatized Mg-Kα X-ray source (1253.6 eV). The source power used was 100 W (10 kV10 mA) and the spectra were acquired in the constant-pass-energy mode at Epas = 44 eV. The overall energy resolution was 1.2 eV as a fullwidth at half maximum (FWHM) for the Ag 3d5/2 line of a pure silver reference. The recorded spectra were fitted using XPS Peak 4.1 software employing Gauss-Lorentz curves after subtraction of a Shirley-type background. The samples were dropcasted above the sample holder from a suspension in DCM, dried under a stream of nitrogen and introduced in the UHV system via a loadlock under inert gas (N2) flux, in order to minimize the exposure to air contaminants and kept in the introduction chamber for at least 12 hours before the measurements. Fe and Al filters were used to reduce unwanted fluorescence and scattered radiation, they were placed as close as possible to the detector window. A Pd foil placed between the second and third ionization chamber was used as an energy calibration reference, a first ionization chamber placed before the sample was used to normalize the fluorescence signal. XAS data were reduced and analyzed with the ATENA/ARTEMIS codes S8 whereas the theoretical XAS signals were generated with the FEFF-8.4 code S9 using muffin tin potentials with the electron densities calculated with a Self Consistent routine and the Hedin-Lundqvist approximation for the potential energy-dependent part. Structural parameters were obtained by data fits in R space with the transformation ranges in k space varying from case to case and a k 2 weighing factor. Sample preparation: all materials were manipulated under argon inside a glove box. The samples to be analyzed were diluted with h-BN, transferred inside a die and made into pellets with a press. Each pellet was fixed to the sample holder using Kapton tape (see Figure S21). During data acquisition, the sample holder was covered with a plastic bag continuously fluxed with argon to keep it under inert atmosphere for the whole measurement. Figure S22. Sample holder with mounted sample, sticked between Kapton tape (orange). A plastic glove transparent to X-rays continuously fluxed with argon (not shown in the figure) was put over the sample holder during data acquisition.